This invention relates to a DC motor, and more particularly, to a flat brushless DC motor in which the rotor includes a field magnet and the stator includes a coil located in a plane parallel to and opposing the lower surface of the magnet.
It is generally known that the torque-angular velocity characteristic of a DC motor is of a "dropping" nature, that is, the generated torque of the motor decreases with increasing angular velocity. If a mechanical viscosity factor .mu. is defined as the reciprocal of the slope of the torque-angular velocity characteristic cirve, a DC motor having a large mechanical viscosity factor .mu. will generally be superior in performance. Thus, for example, a DC motor having a large mechanical viscosity factor .mu., will have superior speed control, that is, it will take less time for the motor to change from one speed to another due to load variations. In addition, a motor having a higher value of .mu. will produce a larger torque and thus be more powerful. Further, the working current and, consequently, the consumed electric power for a given drive voltage, can be reduced by increasing the value of .mu.. Accordingly, a motor having a large value of .mu. has enhanced operating characteristics.
However, DC motors of the described type have generally been designed by a trial and error method and no attempts have been made to accurately increase the mechanical viscosity factor .mu. to optimize the operating characteristics of the motor. Thus, these DC motors are typically quite inefficient in operation.